Most AC-DC power converters use input rectifiers, often as a full wave bridge, sometimes as a voltage doubler, or as both, with switching between modes. The input rectifiers contribute significantly to losses in the converter.
Many AC-DC power converters employ power factor correction. This is often accomplished with two stages in series, a boost converter input stage and a buck converter second stage.
One embodiment of this invention teaches that the input rectifiers in an AC-DC power converter are not needed. A transformer isolated power converter with full wave rectification in the secondary is not sensitive to the polarity of the current in the primary. By using AC switches, the primary may operate with either polarity of input. Thus, the opposite polarity half cycles of commercial power can be accommodated.
It is well known to control voltage and/or current by using Pulse Width Modulation (PWM) in the input side (primary) switches. In the present invention, if the power switches in the primary circuit of a transformer are pules width modulated so that the input current is controlled to be a sine wave, or Power Factor Corrected (PFC), the full wave rectified current out of the secondary of the transformer will be a full wave rectified sine wave, with a value equal to the DC output current (neglecting losses). At the peaks of the full wave rectified sine wave, when the input current is excessive, the excess input current can be shunted to a storage capacitor so that the correct DC current is provided to the output. At the valleys of the full-wave rectified sine wave, when the input current is insufficient, the required additional current can be provided from the storage capacitor, so the correct DC current is provided to the output.
Because the losses of the input rectifier have been eliminated, and because most of the power is carried through a single stage, directly to the output, the present invention is a very efficient high PFC AC-DC converter. This configuration also has the advantage, being a buck converter, that the input current can be controlled from the time power is applied. An arbitrarily long soft start ramp can be used. There is no inrush current, a serious problem of PFC boost converters.
Most of the input power is directed to the output, with the power factor correction storage capacitors and energy control being in a parallel path. The only prior art of which I am aware that uses a parallel path is disclosed in U.S. Pat. Nos. 5,132,606 and 5,144,222, the specifications of which are included herein by reference. In these patents, a parallel path is used, but the parallel path is from the input. This invention teaches a parallel path for control and storage of energy that is entirely on the secondary side of a transformer. Thus the energy storage and the output voltage can be controlled with a secondary side controller. Only the feed back to adjust the input current scaling needs cross the isolation barrier. The referenced patents teach control methods that may be useful for the present invention, however, the present invention does not require these methods. Well known methods of power factor correction control can be adapted to this invention, and the method of control is not at the heart of this invention.